This invention relates generally to a liquid electrophotographic apparatus, and more specifically, to improvements in cleaning a development electrode in a liquid electrophotographic apparatus.
A typical liquid electrophotographic apparatus includes a photoconductor and a development electrode. The development electrode provides an electrical potential which, together with the potential on the photoconductor, provides an electrical field which serves two functions: first, the electrical field urges toner toward the latent image on the photoconductor, secondly, the electrical field urges toner from the non-image areas of the photoconductor toward the electrode and thereby cleans the background (i.e. non-image) areas on the photoconductor. Development electrodes have achieved widespread acceptance in the electrophotographic industry because they can provide a continuous, even coat of toner to the entire image area, including the solid image areas.
One disadvantage of the prior art biased development electrode is that the development electrode itself gets plated with toner while it is performing its cleaning function, thereby rendering it coated with a thick layer of toner particles and impeding the performance of its aforementioned functions. In the prior art, the development electrode has been cleaned by reversing the electrical bias on the development electrode between copies and thereby repelling the charged toner particles toward the surface of the photoconductor. Thus, in a known liquid electrophotographic apparatus, when a latent image on the photoconductor surface is passing the development electrode, an electrical potential having the same polarity as the photoconductor is applied to the development electrode. The potential applied to the development electrode is such that it attracts toner particles from background areas of the photoconductor but is not strong enough to remove toner particles from the image areas on the photoconductor surface. When the image area on the photoconductor has passed from the development electrode region, the bias on the development electrode is changed so as to repel the charged toner particles which had been deposited on the development electrode pushing them back onto the photoconductor surface. Thus, the known cleaning method is based on the fact that the toner deposited on the development electrode can be removed by changing the attractive force to a repulsive force. The toner particles are subsequently removed from the photoconductor surface at a cleaning station located further downstream in the electrophotographic cycle with respect to the movement of the photoconductor.
Prior art systems and methods for cleaning a development electrode suffer several disadvantages. First, the cleaning station has a work very hard to remove the large quantity of toner which was repelled from the development electrode and placed on the photoconductor surface. Secondly, in the prior art systems, it is impossible to run copies continuously, because those systems require a dead band (i.e. an area without an image) on the photoconductor surface between copies. This is especially disadvantageous in an electronic printer or in a very high speed web copier where there is no dead band and copies advantageously follow one immediately after another.
Furthermore, by requiring the cleaning station to remove a large quantity of toner from the photoconductor surface, prior art systems make the changing of toner colors very cumbersome and time consuming.